Water cannon would be of no use in tackling the disturbances seen across Britain, an expert in policing Northern Ireland riots has said.

Jimmy Spratt, a former head of Northern Ireland's police federation and a former political representative on the region's police scrutiny body, said he was surprised by the calls to deploy the crowd control device.

The Democratic Unionist said police in Belfast used water cannon to tackle mobs gathered in open locations and in large numbers.

But he argued that trying to use water cannon around commercial centres in London, Manchester or other cities was not feasible.

He said: "It is not workable at all. There are several points, including the size of these vehicles.

"The big issue would be manoeuvrability. They are suited to operating in a wide space, where police are faced with a crowd of rioters."

He said police in London and elsewhere faced the challenge of tackling relatively small gangs who have moved quickly between shopping centres.

"They are not standing protesting or rioting in one place," said Mr Spratt.

He said that, for the same reason, it may be difficult for police to make effective use of baton rounds on looters, as has also been proposed by some commentators.

The Police Service of Northern Ireland said there were no plans to deploy its water cannon in Britain.